1. Field of the Invention
The invention relates to a direct methanol fuel cell (DMFC), an apparatus consisting of several DMFCs, and a method for operating DMFC apparatuses, with a high voltage efficiency and Faraday efficiency.
2. Description of the Related Art
The principle of the DMFC has been known since 1922; until now work has concentrated on the operation of the DMFC with liquid fuel. In the DMFC, methanol is taken as fuel; in earlier years alternatives to methanol were tried, such as formic acid, formaldehyde or higher-chained alcohols. The use of methanol thereby has the greatest technical significance, for which reason also the name DMFC has become accepted. The operation of the DMFC with liquid fuel takes place at relatively low temperatures, and has the disadvantage that the conversion of the methanol takes place with a relatively poor voltage efficiency, due to kinetic inhibitions of the anode reaction.
The conversion of vaporized methanol is known from J.P.-22 34 359. The water for the moistening of the membrane and for the reaction sequence is thereby supplied separately at the back side (i.e., at the cathode side). The cathode-side supplying of the water has the disadvantage that at higher current densities the electro-osmotic water transport, which is proportional to the current, works against the water diffusion through the membrane. This causes increased water consumption, because the membrane has to be kept moist with additional water. Moreover, the metered addition of water does not take place in load-dependent fashion in this prior art.
A general problem in the operation of the DMFC remains the diffusion of the fuel methanol through the electrolytes to the cathode, where it is also converted. The consequence of this, besides the loss of fuel (lowering of the Faraday efficiency), a reduction of the cell voltage (lowering of the voltage efficiency).
An object of the present invention is thus to provide a fuel cell and a fuel cell apparatus, as well as a method for operating the apparatus, in which high voltage efficiency and high Faraday efficiency are realized at high current densities. In addition, an object of the present invention is that a fuel cell, a fuel cell apparatus, and a method for operating a fuel cell are provided that operate with low electro-osmotic water loss in the cell and with the lowest possible water transport through the polymer electrolytes.
As Faraday efficiency, the degree of energy use is designated that indicates what percentage of the fuel was actually converted at the anode.
As voltage efficiency, the ratio between cell voltage under current loading and thermodynamic rest voltage is designated.
The general recognition of the invention is
First, that an increase of the Faraday efficiency is possible by minimization of the methanol diffusion inside the cell if the methanol is supplied in load-independent fashion and is consumed correspondingly in the anode chamber. It is then not present in a concentration so high that a large diffusion pressure toward the cathode arises.
Second, the invention is based on the recognition that the voltage efficiency can be improved by increasing the operating temperature, because this causes a minimization of the kinetic inhibition of the anode reaction. Moreover, the Faraday efficiency is also increased by the load-dependent supplying of the reactands at a low current density.
Third, the problem of a too-high water transport through the polymer electrodes can be reduced by the addition of an inert gas such as carbon dioxide and/or nitrogen, because by this means the water content at the anode side of the DMFC is lowered, and less water is transported to the cathode.
The subject matter of the present invention is thus a DMFC comprising a supply duct and a waste removal duct for the fuel and the oxidant respectively, a membrane electrode unit and bipolar plates, whereby an evaporating apparatus is connected before the supply duct for the fuel in such a way that, during the conversion, the fuel is present in gaseous form at the anode of the fuel cell. In addition, the subject matter of the present invention is a fuel cell apparatus that comprises a cell stack of inventive fuel cells, an evaporation apparatus and, if warranted, up to three pumps (two dosing pumps for the supply of methanol and water and one pump that brings the CO2 exhaust conducted in the circuit back to the required excess pressure) in the supply line of the fuel as well as a CO2 separator in the drainage line of the fuel, whereby, in the CO2 separator which is connected after the fuel cell stack, the condensate of the gaseous fuel can be separated from the carbon dioxide thermally or in some other way.
In addition, the subject matter of the present invention is a method for operating a DMFC apparatus in which the fuel, consisting at least of methanol and water, is supplied to the anode in gaseous form.
The fuel of the inventive fuel cell can consist either of methanol only or of an arbitrary mixture of water and methanol. If the fuel consists of an arbitrary mixture of water and methanol, then the concentration either of methanol or water can be adjusted in load-dependent fashion via a dosing pump connected before the evaporation apparatus. The fuel can thereby be introduced into the fuel cell with variable pressure, and an arbitrary mixture of inert carrier gas, such as CO2, N2, argon, etc., can be mixed with it.
A preferred embodiment of the fuel cell provides that an inert gas, such as e.g. carbon dioxide and/or nitrogen or the like, can be mixed with the methanol/water mixture. By this means, the water content is reduced at the anode side of the DMFC, and less water is transported through the polymer electrodes to the cathode side.
The degree of moistness xf=VnW/Vn, where VnW=water vapor volume under normal conditions; Vn=total volume under normal conditions, can be adjusted arbitrarily by means of the inert gas. Degrees of moistness greater than 70%, preferably between 80 and 90%, prove useful, because then the polymer membrane is not yet dried out. The degree of moistness will be as high as possible so that the energy expenditure for the gas transport remains as low as possible. The degree of moistness also depends on the operating temperature of the DMFC. The higher this is, the higher the degree of moistness must also be, since the water content in the membrane decreases rapidly at temperatures above 100xc2x0 C. The degree of moistness xf (in relation to the volume) is defined as follows:
xe2x80x83xf=VnW/Vn=VnW/(VnW+VnL)=pW/p
VnL=dry gas volume under normal conditions, i.e. the volume of gaseous methanol, with or without inert gas additive;
pW=water vapor partial pressure
p=total pressure
The inventive fuel cell apparatus preferably consists of a cell stack of inventive fuel cells, but it can also be constructed of various types of fuel cells in combination. The evaporation apparatus and, if warranted, one or two dosing pumps that supply the fuel or the water in load-dependent fashion are thereby integrated into the supply line of the fuel to the cell stack.
In an anode circuit, in the CO2 separator the CO2 that has arisen is separated from the exhaust gas, which is rich in unconsumed methanol. The exhaust gas is then present in condensed form and can be supplied in the circuit, i.e. can be introduced into the evaporation apparatus. In addition, a part of the separated carbon dioxide can likewise be conducted in the circuit via an excess pressure pump that also regulates the amount of added inert gas.
Anode circuit means that the fuel, methanol or methanol/water mixture, respectively with or without inert gas additive, is conducted past the anode in a circular closed system, whereby additional fuel is supplied to the system as needed, and gaseous reaction product is separated out from the system.
The unconsumed fuel contained in the fuel exhaust gas is first condensed or cooled using heat, and is then introduced again into the supply line or into the evaporation apparatus. The load-dependent controlling of the dosing pumps that regulate the inflow of water/methanol into the evaporation apparatus must thereby be constructed in such a way that the changes in concentration of the methanol/water mixture in the evaporation apparatus due to the supplying from the exhaust gas are taken into account.
The unconsumed fuel from the fuel exhaust gas is separated from the contained carbon dioxide physically, or in some circumstances, also chemically in the heat exchanger or CO2 separator. Physical separation thereby means that the separation takes place via the various physical characteristics of the substances (such as density, boiling point, etc.). Chemical separation is also conceivable, including means that the CO2 is chemically bound and precipitated e.g. as carbonate (not very useful energetically, due to the high mass of the resulting carbonate, but alternative chemical methods can be discussed).
As DMFC the direct methanol fuel cell is designated that consists of an anode, a cathode and a suitable electrolyte, in analogy to the general principle of electrochemical energy converters. In general, the electrodes are contacted at the back side, i.e. with the side facing away from the electrolyte, through a current collector which has in addition the task of gas distribution or, respectively, reactand distribution. As a result of the type of electrolytes used, there result various possibilities for the realization of a DMFC. In the context of the present invention, preferred acid electrolytes, and thereby in particular acid solid electrolytes, are treated. In general, proton-conducting polymers (electrolyte membranes), which are stable under the corresponding operating conditions, are thereby suitable. As an example, NAFION (registered trademark) is hereby mentioned as a suitable polymer. As a further electrolyte apart from those mentioned, those based on inorganic systems are hereby also mentioned, such as tin phosphates or electrolytes based on siloxane frameworks.
As current collectors, materials based on carbon, e.g. carbon fiber paper or tissue, are standardly used. As catalysts, at the anode side platinum/ruthenium alloys are used with first priority; at the cathode side pure platinum is mostly used. In the realization of a fuel cell apparatus, such as e.g. a battery, in order to achieve higher voltages the individual cells are connected in series in bipolar fashion. The bipolar plates required therefor can be made of graphite, metallic or other electrically conductive and corrosion-resistant materials. The bipolar plates simultaneously take over the task of reactand supplying. They are thus structured with corresponding ducts, if necessary.
According to the boiling point of the mixture, the operation of the DMFC can take place at temperatures between 60 and 160xc2x0 C. The operating temperature will preferably fall into a range from 100 to 150xc2x0 C.; typically it is between 120 and 130xc2x0 C. Correspondingly, methanol or also corresponding methanol/water mixtures are heated above the boiling point and supplied to the cell in gaseous form. The system pressure is thereby adjusted so that it corresponds to the equilibrium pressure of the methanol/water mixture at the temperature of the fuel cells. In the anode chamber of the DMFC, the vapor is thus in a saturated state. By means of this vaporized supplying of the reactands, the electro-osmotic water transport is minimized, because the quantity of water at the anode is greatly reduced. In the context of the present application, the terms xe2x80x9cfuel,xe2x80x9d xe2x80x9cmethanolxe2x80x9d and xe2x80x9cmixture of water and methanolxe2x80x9d always designate a vaporized fuel that contains an indeterminate quantity of inert gas (i.e., from 0% up to a degree of moistness of almost 100). In the case of CO2 as an inert gas, it can be a part of the anode exhaust gas that is again brought to the required excess pressure via a pump and a corresponding control valve (see also FIG. 2) and is conducted in the circuit.
As stated, a methanol/water mixture or pure ethanol, with or without inert gas additive, is used as fuel; however, the invention is not to be limited thereto, if the electrochemical oxidability of other water-soluble organic molecules turns out to be technically profitable. As stated, the fuel is conducted in the circuit via a carbon dioxide separator connected to the exhaust gas line of the fuel cell, which has at the same time the function of separating the resulting carbon dioxide from the rest of the exhaust gas.
As an oxidant, either pure oxygen or air or arbitrary mixtures of these components are designated, whereby the oxidant of the cathode is preferably supplied in a quantity leaner than stoichiometric.
A particular problem of the DMFC is the search for suitable anode materials for the oxidation of the fuel. Thus, besides the named platinum/ruthenium alloys, according to the state of research, various anode materials and catalysts can be used on the anode according to the invention. For example, it is hereby mentioned that under some circumstances it is possible to bring about a slight additional improvement of the activity of the anode in relation to the binary system platinum/ruthenium by adding a third component, such as tin or nickel, to the alloy. The invention is not to be limited to noble metals as catalysts and anode materials or, respectively, cathode materials; rather, catalysts free of noble metals are also conceivable.
The concentration of methanol in the fuel cell mixture in relation to the unvaporized liquid state can be between 0.05 and 5 mol/l. A concentration between 0.5 and 1.5 mol/l is thereby particularly preferred.
As a further operating parameter the pressure is hereby mentioned, which can be between normal pressure and a slight excess pressure and partial vacuum. The above definitions hold for the specification, the explanations of the Figures and the claims.
Other objects and advantages of the present invention will become apparent from reading the following detailed description and appended claims, and upon reference to the accompanying drawings.